For some reason, outbound emails destined to go to Outlook 365 are being blocked by Microsoft at mail.protection.outlook.com, and there appears to be no reason code from Microsoft for this.
This affects ONLY outbound emails going to recipients who’s businesses are using Microsoft Exchange / Outlook 365 as cloud based mail service provider.
Updates
29/11/2022 @ 16:15 This appears to be affecting mail server nodes MX2 and MX3.
29/11/2022 @ 16:30 We have requested Microsoft to unblock these two mail server nodes by completing the following for each node in the cluster. https://sender.office.com/
29/11/2022 @ 21:13 Microsoft have manually unblocked these two mail server nodes as mail is starting to de-queue.
29/11/2022 @ 23:15 All queuing emails are now delivered. No outbound email was lost. All service is now resumed as normal.
